The present invention relates to a transmission system utilizing trellis-coded modulation based on a constellation split up into N sub-sets, and comprising a data source and a data receiver which itself comprises a decision arrangement which arrangement makes it possible to determine the closest point to a received symbol in each sub-set of the constellation by assigning to the received symbol a combination of N points.
The invention likewise relates to a modem comprising such a decision arrangement.
Currently, trellis coding is widely used in the-field of digital modulation. It consists of splitting up the constellation into sub-sets which are selected so that the minimum square euclidian distance separating two points of a same sub-set is maximized. It is thus possible to divide the constellation several times to obtain the desired distance.
Thus, for transmitting Q data bits, P redundancy bits are added while a constellation of 2.sup.Q+P points is used (in practice P is generally fixed at 1): M of the Q data bits are thus utilized by a convolutional coder having a ratio M/M+P (which corresponds to the addition of P redundancy bits) to indicate one of the 2.sup.M+P sub-sets of the constellation; and the Q-M remaining bits are used to determine, in an indicated sub-set, one Of the 2.sup.Q-M points.
The behaviour of a convolutional coder is described by a trellis of which each branch represents the transition from one state to another of the convolutional coder by the transmission of a point of a given sub-set of the constellation.
The Viterbi algorithm is known to be an optimum decoding method for these convolutional codes. Its principle is described in the article "The Viterbi Algorithm" by G. David Forney, published 3 March 1973 in "Proceedings of the IEEE". It may simply be recollected that the first stage of this algorithm consists of determining for each received symbol the symbol which is closest thereto in the sense of the euclidian distance, in each sub-set of the constellation.
Therefore, it is known that for each of the sub-sets of the constellation the distances separating the received symbol from each of the points of the sub-set under consideration are calculated, after which the symbol of the sub-set that corresponds to the minimum distance is selected.
This method makes it necessary to make a large number of calculations directly proportional to the size of the constellation used. For example, if one goes from a transmission rate of 12000 bits/s (which corresponds to a constellation of 8 sub-sets of. 8 points) to a rate of 14400 bits/s (which corresponds to a constellation of 8 sub-sets of 16 points), the number of calculations is doubled. Thus, for high transmission rates the number of calculations to be made rises considerably. Moreover, the symbol corresponding to the minimum distance is to be searched as many times as there are sub-sets.